RF transmitters and receivers have become widely available and deployed for use in many applications including many commercial products for individuals such as cell phones, garage door openers, automobile keyless entry devices, cordless phones and family radios. RF transmitters and receivers are also widely deployed in more complex commercial, safety and military applications. Collectively, the possible existence of many different RF transmissions from so many different types of equipment presents a broadband RF transmission environment.
In light of the increasing large deployment of many different types of RF transmitters and receivers, the particular RF signals and signal protocols that may be present in any particular local area potentially are quite complex.
At times in a particular local area, it is desirable that the RF local receivers be rendered temporarily inactive thus preventing such local RF receivers from initiating transmissions by any associated local RF transmitters or otherwise from initiating any action.
RF jammers have long been employed for temporarily rendering local RF receivers inactive. However, the large deployment of many different types of RF transmitters and receivers has rendered conventional jammers ineffective in a complex broadband RF environment.
Jamming is usually achieved by transmitting a strong jamming signal at the same frequency or in the same frequency band as that used by the targeted local receiver. The jamming signal may block a single frequency, identified as “spot jamming”, or may block a band of frequencies, identified as “barrage jamming”.
Although simple jammers have long existed, technological advances require the development of advanced jamming equipment. Early jammers were often simple transmitters keyed on a specific frequency thereby producing a carrier which interfered with the normal carriers at targeted local receivers. However, such single carrier jammers have become ineffective and easily avoided using, for example, frequency hopping, spread spectrum and other technologies.
Some jamming equipment has used wide-band RF spectrum transmitters and various audio tone transmissions to jam or to spoof local receivers. Other systems employ frequency tracking receivers and transmitters and utilize several large directional antenna arrays that permit directional jamming of targeted local receivers. Often in such arrays, deep nulls in selected directions are provided to minimize the effects of the jamming in those selected directions. The deep null directions are then used to allow wanted communications.
Some jammers feature several modes of operation and several modulation types. For example, such operational modes include hand keying, random keying, periodic keying, continuous keying and “look through”. In the “look through” mode, a special jammer or a separate receiver/transmitter is used to selectively control the keying of the transmit circuit. The “look through” mode can be configured to hard key the transmitter ON at full power output upon detection of a received signal and periodically hard switch the transmitter RF power to OFF. In unkey operations, while the receiver “looks through” to see if there is still a carrier present or, after the transmitter has hard keyed to full output power ON, the RF output of the transmitter is gradually slewed down to a lower level while the receiver “looks through” to detect any carrier activity on the targeted frequency.
In a continuous-wave operation, when a jammer is only transmitting a steady carrier, the jamming signal beats with other signals and produces a steady tone. In the case of single side band (SSB) or amplitude modulated (AM) signals, a howl sound is produced at the receiver. In the case of frequency modulated (FM) signals, the receiver is desensitized, meaning that the receiver's sensitivity (ability to receive signals) will be greatly reduced.
When various types of modulations are generated by a transmitter, the operation is referred to as “Modulated Jamming”. The modulation sources have been, for example, noise, laughter, singing, music, various tones and so forth. Some of the modulation types are White Noise, White Noise with Modulation, Tone, Bagpipes, Stepped Tones, Swept Tones, FSK Spoof and Crypto Spoof.
The jammers that are actually deployed have tended to be either barrage jammers broadcasting broadband noise or CW (continuous wave) signals targeted at specific known signals. Generally, barrage jammers tend to produce a low energy density in any given communications channel, for example a 25 kHz channel, when jamming a broad band of channels. By way of example, a 200 MHz barrage jammer transmitting 100 Watts generally will only have 12 mWatts in any communications channel and this low power level per channel is likely to be ineffective as a jammer. These jammers also tend to jam wanted communications.
There is a class of jammers that record a brief sample of the signal environment, determine the frequencies of the active signals detected and allocate a jammer transmitter to each of the detected signals. CW signals are typically used as the jammer signals. These systems are limited by the number of transmitters available. In a dense signal environment such as found in urban areas, there are not enough transmitters available and the ones that are available tend to be set on existing signals so that typically no transmitters are available for new signals.
In general, there are two classes of signals to be jammed—analog and digital. The digital signals (for example, key fobs, some radios and cordless phones) require the digital bits in the start of message part of the signal to the targeted communication system to be altered enough to prevent the targeted communication system from recognizing the signal.
A typical analog signal is a family radio signal (FRS). Analog signals are more difficult to jam than digital signals. An FRS local receiver responds to incoming RF transmissions by breaking squelch. If anything is detected by the FRS local receiver (noise or signal), the receiver responds by breaking squelch. In some cases, the mere breaking of squelch by the FRS local receiver is a form of communications. At times, it is desired to render the FRS local receiver totally ineffective including preventing it from even breaking squelch. With current jammer systems, the jammer signal itself typically creates enough “signal” or “noise” to cause the FRS local receiver to break squelch and respond. In such a case, the jammer signal itself may cause the FRS local radio to react. Such reaction can be to cause an associated FRS local transmitter to begin transmitting or to cause some other unwanted action.
For FRS operation, two modes are considered: privacy code ON and privacy code OFF. With the privacy code turned ON, it is sufficient for the jammer to interfere with the signal characteristics to prevent squelch. There are various techniques that are effective against these systems. For example, with privacy code ON, the FRS local radio can be effectively jammed with a simple CW tone at the channel center frequency. With privacy code OFF, any energy in-band will break squelch. It is believed that currently there are no effective jammers known for this privacy code OFF mode.
The FRS radio with privacy code OFF is a simple narrowband FM communication system of the type that has been known for many years. In many such systems, such as radios and telephones, the voice signal on transmission is typically band limited to 300 Hz to 3000 Hz and then the band-limited signal is FM modulated and RF transmitted. The RF receivers operate to FM demodulate the received signal and send the demodulated signal to the speakers or other locations. Historically, any signal energy in the 300 Hz to 3000 Hz band will break squelch.
Modern FRS systems are designed so that the receiving radios will break squelch only when analog FM signals are in particular demodulated frequency bands. In operation, the receivers of such systems measure the energy in the receiver FM demodulator output in demodulated frequency bands, for example, from 1 to 3 kHz and from 5 to 7 kHz. For valid voice signals in such systems, there will be high energy in the 1 to 3 kHz band and very low energy in the 5 to 7 kHz band (since in such systems the 5 to 7 kHz band is filtered from the original transmitted message signal). If the ratio of the energy in these two bands (1 to 3 kHz band and 5 to 7 kHz band) is below a threshold, such FRS system radios are designed to assume that the signal energy is not a signal of interest and are designed not break squelch.
A common jammer technique used in the radar field is to capture an individual local transmitter signal for a short period of time, copy the captured signal as a regenerated signal and retransmit that regenerated signal a short period of time later. Such a “regenerative” jammer creates false radar targets that appear as real targets thereby confusing the radar local receivers. In U.S. Pat. No. 6,476,755, a jammer uses time-division multiplexing techniques that permit monitoring received RF local transmitter signals while, in a time-division multiplexing sense, concurrently transmitting RF signals to jam selected transmissions at local receivers. The time-division multiplexing alternately enables the jamming system receiver and transmitter with operation at a frequency higher than the Nyquist rate.
Radar jammers must have the regenerated jammer transmitted signals synchronized with the jammer received signals. The regenerated jammer transmitted signals must look like the original local transmitter signals, that is, look like the jammer received signals received from the local transmitters. The timing characteristics of the regenerated jammer transmitted signals must match, that is, must be synchronous with, the timing characteristics of the jammer received signals. In the case of radars, the jammer received signals and the regenerated jammer transmitted signals are in the form of pulses. The precise timing, structure, modulation and frequency of each regenerated jammer transmitted signal pulse, that is, the timing characteristics of the pulse, must be the same as the timing, structure, modulation and frequency of the jammer received signal pulse. With such precision in the timing characteristics, the regenerated jammer transmitted signals are said to be synchronous with the jammer received signals. When the regenerated jammer transmitted signals are synchronous with respect to the jammer received signals, the local receiver cannot tell the difference between the regenerated signal pulse and a pulse from a real radar target.
To achieve the required precision in timing characteristics for synchronism, each regenerated jammer transmitted signal pulse must be transmitted at exact times after the jammer received signal pulse. If the received radar signal does not have a constant radar pulse repetition interval (PRI), the regenerated signal cannot have a constant PRI. The regenerated PRI must, to a good approximation, match the received signal PRI. Additionally, the jammer system must capture the entire local transmitter pulse. If the regenerated transmitted signal pulse is a fraction of the jammer received signal pulse, the jamming signal transmitted to the local receiver will appear corrupted and effective jamming will not occur.
In general, the operation of the radar jamming signals of the type described requires regeneration of false target pulses that through precise timing, structure, modulation and frequency appear to be true target pulses which confuse the local receivers to the point where the local receivers will not recognize and act on the received jamming signals.
Notwithstanding the advancements that have been made in jamming systems, the broadband RF transmission environment, particularly as it exists as a result of the proliferation of many different types of RF transmitters and receivers, presents a demanding need for more effective jammers.
In light of the foregoing background, there is a need for improved transmitters, receivers and jammers that are effective in local areas, and in particular are effective for RF broadband environments.